This invention relates to article inspection apparatus and in particular though not exclusively to apparatus for locating the center of a fastener in a base material of a different permeability to that of the fastener e.g., bore centering.
One area of useful application of the invention is the aircraft industry In aircraft construction it is common for stressed skins to be attached to supporting structures by fasteners located in countersunk bores in the skins. Many hundreds of such fasteners may be used regularly spaced apart with a pitch interval of 2 or 3 cms. The bores for such fasteners are potential crack propagation sites and hence require regular inspection e.g. by ultrasonic non-destructive test apparatus. Previous methods of test require removal of the fasteners before scanning for cracks with a hand held probe which is time consuming and removal of the fasteners often entails re-boring of the fastener holes because of aviation rules governing the disturbance of vital parts.